The invention relates to a method for the production of fuel from vegetable oil for operating diesel combustion engines.
Diesel combustion engines for passenger cars and in particular for trucks that can be operated purely on vegetable oil are now available. Commercially available series engines are converted for operation with vegetable oil to achieve this purpose. The engines converted in this manner are operated either with cold-pressed vegetable oils from decentralized oil mills or with so-called fully refined oils originating from interregional extraction plants.
By now, a network of about 500 decentralized oil mills has been established in Germany for the production of cold-pressed vegetable oil. These oil mills process approximately 10% of the national rape oilseed production. The vegetable oil produced by these oil mills typically exhibits a phosphorus content of between 5 and 30 ppm, or between 5 and 30 mg/kg of vegetable oil when expressed as a proportion of weight. The sum of the magnesium and calcium content is typically between 10 and 30 ppm.
The fully refined oil mentioned above is a vegetable oil that has typically been extracted from ground up fruits using hexane. The interim product occurring during the extraction is filtered, degummed and bleached. The final product has a phosphorus content of less than 5 ppm. Good fully refined oils have a phosphorus content of between 1 and 3 ppm and a combined calcium-magnesium value of between 1 and 3 ppm as well. Because the so-called fully refined oils are used mostly in the food industry, there are no regulations with regard to maximum values for phosphorus, calcium and magnesium.
Analyses for specifying phosphorus, calcium and magnesium contents of vegetable oils used as fuel have by now led to a pre-standard of the Deutsches Institut für Normung [German Institute for Standards]. This DIN pre-standard DINV 51605 specifies the maximum content of these three substances in a fuel made of vegetable oil as follows:                Phosphorus: 12 ppm        Calcium+Magnesium: 20 ppm.        
When operating engines using fuel according to this pre-standard, it became apparent that unexplainable incrustations often occur on the pistons, deposit themselves on the valve discs or the glow plugs, or block the catalytic converter in the exhaust, or plug up the soot filter. The exact operating conditions under which these deposits occur have thus far not been conclusively clarified.
The same phenomena occur with engines that are operated using rape methyl ester, the so-called biodiesel. Biodiesel is produced pursuant to the DIN standard DIN EN14214. This standard specifies the following maximum phosphorus, calcium and magnesium values:                Phosphorus: 10 mg/kg        Calcium+Magnesium: 5 mg/kg        
To the extent that fully refined oils are manufactured by extraction plants not only for the food industry but also as fuel, the values for phosphorus, calcium and magnesium are steadily approaching the higher values according to the DIN pre-standard for vegetable oil as a fuel. This means that the phosphorus, calcium and magnesium content increases in instances when the fully refined oil is used as a fuel as opposed to its use in the food industry. This also leads to a cost advantage when the extraction rate is increased for fuel production or when purification efforts are decreased.